Rules as written, there's no specific thing telling you when a Vibro-Blade's cell runs out. So, there's a few options a sneaky DM could employ:
Decide it doesn't matter
You can easily just ignore this. Power cells for vibros fairly inconsequential. You'll need to decide how it compares with what your players like to do. If you're not having fun worrying about it? Don't.
Pick a more general milestone
Instead of specific minutes or rounds (which I feel just adds a ton of annoying minutae to the game), track in number of combats. Say, five to seven. This way, you get a realistic duration for your vibro-blades, and don't have to get too into the reeds on tracking.
Save it for the Worst Possible Moment™
This is probably best combined with one of the options above, but as the DM you can always cause the blade to short out, fault, or run out of battery at the most dramatic moment, and enjoy the resulting chaos. Things like this give you a way to acknowledge the reality of future weapons, and inject a sudden, shocking twist into combat. This trick really only works once, though, so pick a good moment to do it, perhaps when your blade-happy mandalorian is seconds away from revenge...
Finally, we've seen in a few movies for Star Wars that the Bayonets run off the power cell in the gun they're attached to (Look at the green pigs in Jabba's Palace). Thus, unless you're going to pull them off the barrel of a blaster, you shouldn't have to worry too much. I'd have a lot of fun with it if a player said "oh, I pull it off and plug a full-sized power pack into it". Well. More fun with the explosion, but you get what I mean.